Transcranial alternating current stimulation
Encyclopedia
Transcranial alternating current stimulation (tACS) is a noninvasive means by which alternating current
s applied through the skull
over the occipital cortex of the brain entrains in a frequency
-specific fashion the neural oscillations
of the underlying brain
.
s are used: a stimulating one over the target cortex, and a reference one elsewhere, such as on the top of the head or on the neck. The size of the stimulation electrode is around 3 x 4 cm and the reference electrode has three times the surface area so as to reduce current density and limit stimulating the skin
. They are held in place by elastic bands, and the hair and skin are saturated with saline
solution for about 5-10 minutes. There is an initial sensation on the scalp
but, after the initial few minutes, this fades.
The alternating current applied is sinusoidal at a voltage
of 5 to 15V. The current density under the stimulation electrode is about 83μA per square cm.
produces its effects depending upon frequency and illumination
levels. When applied in an illuminated room, it induces most effectively the perception of continuously flickering light (phosphene
s) with frequencies in the beta wave
. But when in the dark such perception of flickering lights is most effective when the stimulation frequency when done in alpha wave
range. The phosphenes are reported to be seen in the far peripheral areas of vision
. This suggests that tACS stimulates the anterior part of the visual cortex on its medial wall where the peripheral areas of the visual field
are processed. The reason for this could be that the current flows mostly in the cerebrospinal fluid
between the two cerebral hemisphere
s and enters the cerebral cortex from the forward part of the visual cortex.
No effects are produced by current alternations at the theta wave or gamma wave
frequencies.
on one side brain at beta wave
frequencies entrains activity in this range and increases the coherence between scalp-recorded electroencephalography
(EEG) and electromyographic (EMG) activity in the first dorsal interosseous
muscle of the opposite hand when it is held steady on a joystick. The brain waves led muscle EMG by 41.6 ms suggesting that the synchronization with muscle activity is caused by the brain. There is also a small but significant slowing of the reaction time when a person moves the joystick to a target.
Transcranial alternating current stimulation produces no effects upon motor evoked potential
s or EEG power.
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
s applied through the skull
Human skull
The human skull is a bony structure, skeleton, that is in the human head and which supports the structures of the face and forms a cavity for the brain.In humans, the adult skull is normally made up of 22 bones...
over the occipital cortex of the brain entrains in a frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
-specific fashion the neural oscillations
Neural oscillations
Neural oscillation is rhythmic or repetitive neural activity in the central nervous system. Neural tissue can generate oscillatory activity in many ways, driven either by mechanisms localized within individual neurons or by interactions between neurons...
of the underlying brain
Human brain
The human brain has the same general structure as the brains of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size. Estimates for the number of neurons in the human brain range from 80 to 120 billion...
.
Method
Two electrodeElectrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s are used: a stimulating one over the target cortex, and a reference one elsewhere, such as on the top of the head or on the neck. The size of the stimulation electrode is around 3 x 4 cm and the reference electrode has three times the surface area so as to reduce current density and limit stimulating the skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
. They are held in place by elastic bands, and the hair and skin are saturated with saline
Saline
Saline may refer to:* Salinity, the salt content of a solution** Saline water, water containing significant concentration of salts* Soil salinity, salt content of soil* Saline , a liquid with salt content to match the human body...
solution for about 5-10 minutes. There is an initial sensation on the scalp
Scalp
The scalp is the anatomical area bordered by the face anteriorly and the neck to the sides and posteriorly.-Layers:It is usually described as having five layers, which can conveniently be remembered as a mnemonic:...
but, after the initial few minutes, this fades.
The alternating current applied is sinusoidal at a voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
of 5 to 15V. The current density under the stimulation electrode is about 83μA per square cm.
Visual cortex
Transcranial alternating current stimulation applied over the visual cortexVisual cortex
The visual cortex of the brain is the part of the cerebral cortex responsible for processing visual information. It is located in the occipital lobe, in the back of the brain....
produces its effects depending upon frequency and illumination
Lighting
Lighting or illumination is the deliberate application of light to achieve some practical or aesthetic effect. Lighting includes the use of both artificial light sources such as lamps and light fixtures, as well as natural illumination by capturing daylight...
levels. When applied in an illuminated room, it induces most effectively the perception of continuously flickering light (phosphene
Phosphene
A phosphene is a phenomenon characterized by the experience of seeing light without light actually entering the eye. The word phosphene comes from the Greek words phos and phainein...
s) with frequencies in the beta wave
Beta wave
Beta wave, or beta rhythm, is the term used to designate the frequency range of human brain activity between 12 and 30 Hz . Beta waves are split into three sections: High Beta Waves ; Beta Waves ; and Low Beta Waves...
. But when in the dark such perception of flickering lights is most effective when the stimulation frequency when done in alpha wave
Alpha wave
Alpha waves are neural oscillations in the frequency range of 8–12 Hz arising from synchronous and coherent electrical activity of thalamic pacemaker cells in humans...
range. The phosphenes are reported to be seen in the far peripheral areas of vision
Peripheral vision
Peripheral vision is a part of vision that occurs outside the very center of gaze. There is a broad set of non-central points in the field of view that is included in the notion of peripheral vision...
. This suggests that tACS stimulates the anterior part of the visual cortex on its medial wall where the peripheral areas of the visual field
Visual field
The term visual field is sometimes used as a synonym to field of view, though they do not designate the same thing. The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments", while 'field of view' "refers to the physical...
are processed. The reason for this could be that the current flows mostly in the cerebrospinal fluid
Cerebrospinal fluid
Cerebrospinal fluid , Liquor cerebrospinalis, is a clear, colorless, bodily fluid, that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord...
between the two cerebral hemisphere
Cerebral hemisphere
A cerebral hemisphere is one of the two regions of the eutherian brain that are delineated by the median plane, . The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres has an outer layer of grey matter called the cerebral cortex that is...
s and enters the cerebral cortex from the forward part of the visual cortex.
No effects are produced by current alternations at the theta wave or gamma wave
Gamma wave
A gamma wave is a pattern of neural oscillation in humans with a frequency between 25 to 100 Hz, though 40 Hz is prototypical.According to a popular theory, gamma waves may be implicated in creating the unity of conscious perception...
frequencies.
Motor cortex
Transcranial alternating current stimulation applied over the motor cortexMotor cortex
Motor cortex is a term that describes regions of the cerebral cortex involved in the planning, control, and execution of voluntary motor functions.-Anatomy of the motor cortex :The motor cortex can be divided into four main parts:...
on one side brain at beta wave
Beta wave
Beta wave, or beta rhythm, is the term used to designate the frequency range of human brain activity between 12 and 30 Hz . Beta waves are split into three sections: High Beta Waves ; Beta Waves ; and Low Beta Waves...
frequencies entrains activity in this range and increases the coherence between scalp-recorded electroencephalography
Electroencephalography
Electroencephalography is the recording of electrical activity along the scalp. EEG measures voltage fluctuations resulting from ionic current flows within the neurons of the brain...
(EEG) and electromyographic (EMG) activity in the first dorsal interosseous
Dorsal interossei of the hand
The dorsal interossei of the hand are muscles that occupy the space between the metacarpals.-Structure:There are four dorsal interossei in each hand...
muscle of the opposite hand when it is held steady on a joystick. The brain waves led muscle EMG by 41.6 ms suggesting that the synchronization with muscle activity is caused by the brain. There is also a small but significant slowing of the reaction time when a person moves the joystick to a target.
Transcranial alternating current stimulation produces no effects upon motor evoked potential
Evoked potential
An evoked potential is an electrical potential recorded from the nervous system of a human or other animal following presentation of a stimulus, as distinct from spontaneous potentials as detected by electroencephalography or electromyography .Evoked potential amplitudes tend to be low, ranging...
s or EEG power.
See also
- Transcranial direct current stimulationTranscranial direct current stimulationTranscranial direct current stimulation is a form of neurostimulation which uses constant, low current delivered directly to the brain area of interest via small electrodes...
- Transcranial magnetic stimulationTranscranial magnetic stimulationTranscranial magnetic stimulation is a noninvasive method to cause depolarization or hyperpolarization in the neurons of the brain...
- Brainwave entrainment